The known-heating-cost dividers operating on the evaporation principle consist basically of a bulb filled with a measuring liquid. Such a device is described in detail further below. Such dividers of heating costs have the disadvantage that under the influence of external factors, such as sunlight, proximity to an oven, hot water in the bathroom, etc, and also under the influence of cold evaporation, erroneous values can be given, which results in an erroneous apportionment of the heating costs. Due to the mentioned external influences, more measuring liquid evaporates from the bulb than corresponds to the actual heat supplied by the heating apparatus, which leads to the allotment of a higher fraction of the heating cost. Especially the radiation from external heat sources has proven to be a problem in measuring of the heat amounts.
A certain compensation of the so-called cold evaporation of the measuring liquid (spontaneous evaporation of the measuring liquid within the measuring period without heating of the heat surfaces) has been achieved by filling the bulb with a larger amount of measuring liquid such that the additional volume corresponds to a value established by experience. This type of compensation can give only a rough average value, which does not take into consideration the individual condition of the heated rooms. Further, only the cold evaporation can be compensated in this manner, but not the external influences mentioned above, such as radiation from external heat sources.